ENKEPHALIN-CONTAINING INTERNEURONS ARE SPECIALIZED TO INNERVATE OTHERINTERNEURONS IN THE HIPPOCAMPAL CA1 REGION OF THE RAT AND GUINEA-PIG

Enkephalins are known to have a profound effect on hippocampal inhibit ion, but the possible endogenous source of these neuropeptides, and th eir relationship to inhibitory interneurons is still to be identified. In the present study we analysed the morphological characteristics of met-enkephalin-immunoreactive cells in the CA1 region of the rat and guinea-pig hippocampus, their coexistence with other neuronal markers and their target selectivity at the light and electron microscopic lev els. Several interneurons in all subfields of the hippocampus were fou nd to be immunoreactive for met-enkephalin. In the guinea-pig, fibres arising from immunoreactive interneurons were seen to form a plexus in the stratum oriens/alveus border zone, and basket-like arrays of bout ons on both enkephalin-immunoreactive and immunonegative cell bodies i n all strata. Immunoreactive boutons always established symmetric syna ptic contacts on somata and dendritic shafts. Enkephalin-immunoreactiv e cells co-localized GABA, vasoactive intestinal polypeptide and calre tinin. Postembedding immunogold staining for GABA showed that all the analysed enkephalin-immunoreactive boutons contacted GABAergic postsyn aptic structures. In double-immunostained sections, enkephalin-positiv e axons were seen to innervate calbindin D28k-, somatostatin-, calreti nin-and vasoactive intestinal polypeptide-immunoreactive cells with mu ltiple contacts. Based on these characteristics, enkephalin-containing cells in the hippocampus are classified as interneurons specialized t o innervate other interneurons, and represent a subset of vasoactive i ntestinal polypeptide-and calretinin-containing cells. The striking ma tch of ligand and receptor distribution in the case of enkephalin-medi ated interneuronal communication suggests that this neuropeptide may p lay an important role in the synchronization and timing of inhibition involved in rhythmic network activities of the hippocampus.